In recent years, as semiconductor integrated circuits, such as ICs or LSIs or a liquid crystal panel, are micronized, and their integration degree increases, the accuracy and function of an exposure apparatus, such as a semiconductor exposure apparatus, improve. In particular, in alignment of an original, such as a mask or reticle, and a substrate, such as a semiconductor wafer, a technique, which overlays the original and substrate on the order of several nm to be promising, is expected. As an exposure apparatus used in the manufacture of devices, such as semiconductor integrated circuits, an apparatus, called a stepper or a step and scan, is used often.
This apparatus sequentially transfers a pattern formed on an original (e.g., a reticle) to a plurality of portions of the substrate (e.g., a semiconductor wafer). An apparatus that performs this transfer is collectively called a stepper, and an apparatus that performs this transfer while scanning a stage is called a step and scan.
Alignment of the original and substrate in the exposure apparatus will be described.
An example of alignment of the original and substrate in the exposure apparatus includes a die-by-die alignment scheme, which performs alignment by measuring the exposure position for each exposure. A global alignment scheme is also available, which performs position measurement at an appropriate number of measurement points in advance, and creates a correction equation of the exposure position from the measurement result to perform alignment.
The global alignment scheme is an excellent scheme with which a high throughput and high accuracy can be obtained. According to the global alignment scheme, alignment is performed according to one correction equation for the entire area of the substrate. Hence, the state of alignment can be judged by examining several points of the substrate, leading to an advantage in usage as well. To perform alignment, the alignment target itself or an alignment mark arranged in its vicinity must be detected. An example of a detection scheme for this includes the following two schemes:
1. The TTL (Through The Lens) scheme with which the position of the alignment mark is measured through a projection optical system.
2. The OA (Off Axis) scheme with which the position of an alignment mark is measured directly and not through a projection optical system.
When the original and substrate are to be aligned by the OA scheme, the base line amount as the gap between the measurement center of an alignment detection system and the projection image center (exposure center) of the original pattern must be known.
More specifically, the substrate must be moved to a position which is obtained by correcting the position of a target exposure region (also called a shot region), which is measured by using the alignment detection system, with the base line amount, so that the center of the shot region is correctly aligned with the exposure center. During use of the exposure apparatus, sometimes, the base line amount gradually fluctuates. When fluctuation in the base line amount occurs, the alignment accuracy (overlaying accuracy) decreases.
To prevent this, conventionally, the base line amount is measured periodically.
So far, the conventional exposure apparatus and alignment with the exposure apparatus have been described briefly.
A prior art (first prior art) arrangement of alignment of a wafer and a reticle in a semiconductor exposure apparatus will be described with reference to FIG. 2.
Referring to FIG. 2, reference numeral 1 denotes an illumination optical system; reference numeral 2, a reticle serving as an original; reference numeral 3, a projection optical system; and reference numeral 4, a wafer serving as a substrate. The image of the reticle 2 illuminated by the illumination optical system 1 is projected onto the wafer 4 through the projection optical system 3. Reference numeral 5 denotes a wafer stage; reference numeral 6, a wafer chuck; and reference numeral 7, a wafer stage controller. The wafer 4 is placed on the wafer chuck 6 on the wafer stage 5 by a wafer transport device (not shown). The wafer stage 5 is positioned by the wafer stage controller 7. Reference numeral 8a and 8b denote TTL alignment detection systems, respectively; reference numeral 9, an OA alignment detection system; reference numeral 10, a height detection unit; and reference numeral 11, a controller or processor which controls the exposure apparatus and executes various calculations.
FIG. 7 is a view of the wafer stage 5 when seen from the direction of the projection optical system 3. The wafer stage 5 has a reference mark 12, equivalent to an alignment mark, formed on the surface of the wafer 4, or a reference member 12 having such a reference mark, at a position where it does not interfere with the wafer 4, as shown in FIG. 7.
The reticle 2 has marks RMa and RMb at positions symmetrical with reference to a center C, as roughly shown in FIG. 8. The reticle 2 is held on a reticle stage (not shown). The reticle stage moves the reticle 2 to a position where the center C coincides with an optical axis AX of the projection optical system 3.
The wafer stage 5 is positioned such that the reference mark on the wafer stage 5 falls on a predetermined position in the projection field of the projection optical system 3. Then, the TTL alignment detection system 8a provided above the reticle 2 can detect the mark RMa of the reticle 2 and the reference mark simultaneously. When the wafer stage 5 is moved to another position, the TTL alignment detection system 8b can detect the mark RMb of the reticle 2 and the reference mark simultaneously. The OA alignment detection system 9 is fixed outside the projection optical system 3 (outside the projection field). An optical axis OX of the OA alignment detection system 9 is parallel to the optical axis AX of the projection optical system 3.
The exposure method of the first prior art exposure apparatus will be described with reference to FIG. 9.
Referring to FIG. 9, step S301 is a base line measurement step. The position of the wafer stage 5, obtained when the mark RMa of the reticle 2 and the reference mark on the reference member 12 are aligned by using the TTL alignment detection systems 8a and 8b, is measured by an interferometer, such as a laser interferometer (not shown).
Similarly, the position of the wafer stage 5, obtained when the mark RMb of the reticle 2 and the reference mark on the reference member 12 are aligned by using the TTL alignment detection systems 8a and 8b, is measured by the interferometer, such as the laser interferometer described above.
The reference mark on the reference member 12, obtained when the wafer stage 5 is located at the central position (average value) of the position of the wafer stage 5 with respect to the marks RMa and RMb, is on the optical axis AX of the projection optical system 3, and located at a position conjugate with the center C of the reference member 12.
Similarly, the position of the wafer stage 5, obtained when the reference mark on the reference member 12 is aligned with the OA alignment detection system 9, is measured by the interferometer, such as the laser interferometer described above. A base line amount BL is obtained by calculating the difference between the central position (position of the optical axis AX) (described above) detected by the TTL alignment detection systems 8a and 8b and the position (position of the optical axis OX) (described above) detected by the OA alignment detection system 9.
Step S302 is a wafer pattern position measurement step. In the wafer pattern position measurement step, the position error amount of the pattern (wafer pattern) on the wafer 4 is measured with reference, as an origin, to the position to which the wafer stage 5 has been moved from the exposure center position by the base line amount BL measured in the base line measurement step.
More specifically, the positions of a plurality of alignment marks on the wafer 4 are measured by the OA alignment detection system 9 to create a correction equation for global alignment. Namely, the shift, magnification offset, rotation, and the like, of the wafer pattern, are measured. As a conventional example of the wafer pattern position measurement step, for example, one proposed by Japanese Patent Laid-Open No. 9-218714 is available.
Japanese Patent Laid-Open No. 9-218714 proposes an example of the global alignment scheme. Particularly, a high-order error factor is also corrected to improve the alignment accuracy. A simple example of the correction equation includes the following equations (1) and (2):dwx=Mwx*x+θwx*y+Swx  (1)dwy=θwy*x+Mwy*y+Swy  (2)where dwy and dwy are the position error amounts at a coordinate point (x, y) on the wafer 4. An X-direction magnification offset Mwx, Y-direction magnification offset Mwy, X-direction rotation θwx, Y-direction rotation θwy, X-direction shift Swx, and Y-direction shift Swy of the pattern on the wafer 4 as the respective coefficients can be obtained by the method of least squares.
Step S303 is an exposure step. In the exposure step, the wafer stage 5 is driven on the basis of the base line amount and the position error amount of the pattern on the wafer 4, which is measured in the wafer pattern position measurement step, to transfer the pattern of the reticle 2 onto the wafer 4.
So far, the exposure apparatus and alignment of the wafer and reticle according to the first prior art arrangement have been described.
An exposure apparatus and alignment of the wafer and reticle according to another prior art (second prior art) arrangement will be described.
As described above, the ICs and LSIs have been rapidly shrinking in feature size, and each year, a higher apparatus performance is demanded for a semiconductor manufacturing apparatus. In recent years, an improvement in productivity is strongly sought to accompany the increasing demands for semiconductors represented by DRAMs. Thus, not only an increase in accuracy, but also, an increase in throughput, is demanded for the semiconductor manufacturing apparatus.
In view of this, Japanese Patent Publication No. 1-49007 B2 separately has a function (to be referred to as a measurement station hereinafter) for measuring a pattern position on a wafer and a function (to be referred to as an exposure station hereinafter) for exposing the wafer to light. More specifically, an exposure apparatus, which performs a measurement process and exposure process simultaneously, is proposed. As an example of this exposure apparatus, the second prior art arrangement will be described with reference to FIG. 5.
The exposure apparatus according to the second prior art arrangement includes a measurement station 13, an exposure station 14, a wafer supply unit 15, and a controller 11. The measurement station 13 measures the relative positional relationship between a wafer chuck serving as a wafer support and a pattern on a wafer. After the relative positional relationship between the reticle and wafer chuck is measured, the exposure station 14 projects the pattern of the reticle to the wafer and exposes the wafer to light. The wafer supply unit 15 transfers the wafer and wafer chuck between the measurement station 13 and exposure station 14. The controller 11 controls the measurement station 13, exposure station 14, and wafer supply unit 15.
In the measurement station 13, reference numeral 9 denotes an OA alignment detection system; reference numeral 4a, a wafer serving as a target exposure substrate; reference numeral 6a, a wafer chuck; reference numeral 5a, a wafer stage; and reference numeral 10, a height detection unit. The wafer chuck 6a serves as a substrate support which mounts and holds the wafer 4a on it. The wafer stage 5a mounts the wafer chuck 6a on it and its position is measured by a stage controller 7a to position the wafer 4a. 
In the exposure station 14, reference numeral 3 denotes a projection optical system; reference numerals 8a and 8b, TTL alignment detection systems; reference numeral 1, an illumination optical system; and reference numeral 5b, a wafer stage. The projection optical system 3 projects the image of a reticle 2 onto a wafer 4b. The position of the wafer stage 5b is measured by a stage controller 7b, which positions a wafer chuck 6b on which the wafer 4b is mounted.
FIG. 10 is a view of the wafer chuck 6b when seen from the direction of the projection optical system 3. Reference members 12a and 12b are fixed to each of the wafer chucks 6a and 6b, respectively having reference marks equivalent to the alignment marks formed on the surfaces of the corresponding wafers 4a and 4b, at positions where they do not interfere with the wafer 4a, as shown in FIG. 10.
According to this prior art arrangement, the pattern of the reticle is exposed to the wafer in the following procedure.
First, in the measurement station 13, the alignment mark positions on the wafer chuck 6a and wafer 4a are measured by using the OA alignment detection system 9 to measure the relative positional relationship between the wafer chuck 6a and the pattern on the wafer 4a. At this time, in the exposure station 14, the wafer 4b is exposed to light simultaneously with the procedure to be described hereinafter.
Subsequently, the exposed wafer 4b and wafer chuck 6b are unloaded from the exposure station 14 by using the wafer supply unit. The wafer 4a and wafer chuck 6a of the measurement station 13 are supplied to the exposure station 14.
In the exposure station 14, the alignment mark position on the wafer chuck 6b is measured by the TTL alignment detection systems 8a and 8b through the reticle 2 to measure the relative positional relationship between the pattern on the reticle 2 and the wafer chuck 6b. In addition, the relative positional relationship between the patterns on the reticle 2 and wafer 4b is calculated by using the relative positional relationship between the wafer chuck 6a and the pattern on the wafer 4a measured in the measurement station 13. Finally, the pattern of the reticle 2 is exposed onto the wafer 4b on the basis of the calculated relative positional relationship between the calculated patterns on the reticle 2 and wafer 4b. 
According to this prior art arrangement, the process of the measurement station 13 and that of the exposure station 14 can be performed simultaneously, so that the total processing time of accurate alignment and wafer exposure can be shortened.
A case has been described wherein the wafer chuck is used as a substrate which supports the wafer when the wafer is to be moved between the measurement station 13 and exposure station 14. However, the present invention is not limited to this. For example, the wafer stages 5a and 5b can be used as substrate supports when the wafer is to be moved. In this case, in place of detecting the alignment mark on the wafer chuck, the alignment marks on the wafer stages are detected in the same manner.
The exposure method of the exposure apparatus according to the second prior art arrangment will be described with reference to FIG. 11.
Referring to FIG. 11, step S401 is a measurement position chuck mark position measurement step. In step S401, the reference mark positions on the reference members 12a and 12b on the wafer chuck 6a are measured by using the OA alignment detection system 9. As shown in FIG. 10, the wafer chuck 6a has, as alignment marks, reference marks at least on the two reference members 12a and 12b. These reference marks are measured by the OA alignment detection system 9. Thus, the position and rotation amount of the wafer chuck 6a, with respect to the OA alignment detection system 9, are measured.
Step S402 is a wafer pattern position measurement step. In step S402, the alignment mark position on the wafer chuck 6a is measured in the measurement station 13 by using the OA alignment detection system 9. Thus, the position of the pattern (wafer pattern) on the wafer 4a is measured. The wafer pattern position measurement step is identical to that of the first prior art arrangement described above, and a detailed description thereof will be omitted. The relative positional relationship between the wafer chuck 6a and the pattern on the wafer 42 is measured by the measurement position chuck mark position measurement step and the wafer pattern position measurement step.
Step S403 is an exposure position chuck mark position measurement step. In step S403, the positions of the reference marks on the reference members 12a and 12b of the wafer chuck 6b are measured in the exposure station 14 by the TTL alignment detection systems 8a and 8b through the reticle 2. Thus, the relative positional relationship (position and rotation amount) between the pattern on the reticle 2 and the wafer chuck 6 b is obtained.
Step S404 is an exposure step. In step S404, the relative positional relationship between the pattern on the reticle 2 and the pattern on the wafer 4b is calculated by using the relative positional relationship between the wafer chuck 6a and the pattern on the wafer 4a, which is measured in steps S401 and S402 and the relative positional relationship between the pattern on the reticle 2 and the wafer chuck 6b, which is measured in step S403. The wafer stage 5b is driven on the basis of the calculated relative positional relationship to transfer the pattern of the reticle 2 onto the wafer 4b. 
So far, the exposure apparatus and alignment of the wafer and reticle according to the second prior art arrangement have been described.
The prior art arrangements discussed above enable highly accurate alignment of the reticle and wafer, with a high throughput.
In recent years, a further demand has arisen from an improvement in alignment accuracy, and accordingly, an error component, which is conventionally regarded as an infinitesimal amount, has become non-negligible. For example, if the wafer stage drive characteristic differs between the measurement position and exposure position of the first prior art arrangement, an alignment error can occur. Similarly, if the wafer stage drive characteristic differs between the measurement station and exposure station of the second prior art, an alignment error can occur. Such a difference in wafer stage drive characteristic will be referred to as a stage drive characteristic difference.
Conventionally, in the manufacture of the exposure apparatus, the stage drive characteristic at the measurement position or measurement station and that at the exposure position or exposure station are adjusted such that the error component becomes an infinitesimal amount. In addition, the error component, including the stage drive characteristic difference, is measured in advance, and corrected as an offset.
If, however, the stage drive characteristic difference changes because, e.g., the stage drive characteristic changes over time, an alignment error can occur, even if it is infinitesimal. Such an error can become non-negligible in meeting the future demand for an improvement in alignment accuracy.